There exist many applications for laser diodes and arrays of laser diodes that can emit TM-polarized light or simultaneously or selectively emit separable light from different array elements. Examples are color printing, full color digital film, recording, color displays, and optical recording and playback systems. Building a laser array into a monolithic structure in which the emitting regions are closely spaced offers a further important advantage that all of the optical sources can share a common lens system. It is also desirable in a number of these applications that the emitting regions in the monolithic structure are individually addressable, and it is also desirable to be able to individually detect such closely-spaced beams and process any information contained as a result of beam modulation at the source or by reflection from or transmission through optical media.
There are several ways to generate from a laser diode light beams with a unique characteristic. One way is to control its polarization.
The first above-referenced related application, Ser. No. 07/994,029, whose contents are herein incorporated by reference, describes a structure for generating dual polarization beams from a monolithic, addressable, laser diode chip. The structure comprises a stack of epitaxially-deposited layers including multiple, vertically-stacked QW heterostructures of similar or different compositions but capable of generating light that is differently polarized. The different QW structures can be at different levels. By selective etching and/or diffusion techniques, laterally-spaced different QW structures can be isolated and separately activated for individual addressing.
The second and third above-referenced related applications, Ser. Nos. 07/948,524 and 07/948,522, whose contents are herein incorporated by reference, describe the construction and method of manufacture on a common substrate of individually addressable quantum well (QW) lasers that can be caused to oscillate in the transverse electric (TE) or in the transverse magnetic (TM) mode, and QW laser constructions that can be switched from oscillating in the TE polarization mode to the TM polarization mode, or vice versa. This has been accomplished in certain material systems by controlling the type of strain induced in an epitaxially deposited active region due to lattice mismatches with the substrate. Thus, in most material systems allowing heavy hole and light hole transitions, when the n=1 heavy hole is the lowest energy state and therefore the state whose population is most easily inverted, usually true for unstrained and compressively strained III-V alloy systems, TE polarization gain will predominate. However, by reversing the light hole and heavy hole band edges, achieved in certain material systems by inducing tensile strain into the active region, TM polarization gain will predominate. In the degenerate condition, where the light hole and heavy hole bands are substantially coincident in energy, the polarization of the emission can be determined by threshold carrier density and other factors, such as temperature, facet reflectivity, cavity length and intracavity optical loss.
In general, the desired result of controlling polarization can be achieved with either a single QW, carefully adjusted, or separate Qws for TE and TM mode gain, respectively, with the polarization mode of laser oscillation dependent upon the gain characteristic and the threshold gain. The necessary gain characteristic has one polarization with lowest transparency current, and the orthogonal polarization with a greater peak gain. For some range of active region parameters (thickness, composition, placement within the confining region, etc.), these characteristics can be obtained, and so the polarization will be determined by the threshold gain. Therefore, the polarization of each device can be selected, for example by introducing an additional loss into one of the devices, thereby forcing it to oscillate in the higher-gain polarization. On the other hand, a device without this additional loss will simply oscillate in the polarization which has the lowest transparency current. The additional loss could be an unpumped section along the cavity, low mirror reflectivity, shorter cavity, etc. Similarly, the polarization of each device could be switched, by using an intracavity loss modulator. This polarization selectivity mechanism is demonstrated by the polarization-dependent gain-current characteristics shown in FIG. 1.
In the graph of FIG. 1, the abscissa represents current and the ordinate represents gain of an active layer having the characteristics described above, with the curve 10 labelled TE showing that, under certain conditions, the QW when caused to lase at the lower threshold 11 will emit TE-polarized radiation. Under other conditions, usually by introducing loss, when the threshold current exceeds the crossover 12, the QW will lase in the higher gain, TM mode represented by the curve 13.
Applying the principles described above to the material systems disclosed in the referenced prior applications resulted in laser devices that could emit in the spectral range below about 850 nm. However, there are important applications for such laser devices that emit significantly above the 850 nm limit.